1. Field of the Invention
The present invention relates to a GaN-based semiconductor device, and more particularly, to a GaN-based semiconductor device having a novel layered structure and suited for use as a high-electron mobility transistor (HEMT), a field-effect transistor (FET), etc.
2. Description of the Related Art
GaN-based semiconductor materials, such as GaN, InGaN, AlGaN and AlInGaN, have high band-gap energy as well as high dielectric breakdown field strength, compared with Si-based or GaAs-based materials, for example. Also, GaN-based materials ensure superior high-temperature operation and have high electron saturation velocity.
Currently, therefore, diligent study and development of electronic devices using GaN-based materials, such as HEMT and FET, are under way, and because of the above superior characteristics, GaN-based materials are attracting attention as a useful material for power devices handling microwave or milliwave band. Namely, researches and development of novel small-sized, high reliability, low-loss devices using GaN-based materials are in progress, and it is essential for this type of semiconductor devices to have high gate-drain breakdown voltage, increased current density of operating layer and low-resistance contact electrodes.
A GaN-based HEMT structure shown in FIG. 1, for example, is conventionally known. As illustrated, the conventional HEMT has a layered structure wherein a buffer layer of GaN, an undoped GaN layer and an undoped AlGaN layer are successively formed on a semi-insulating substrate of sapphire, for example. A source electrode S and a drain electrode D are formed on the surface of the undoped AlGaN layer so as to establish ohmic contact with a contact layer of, for example, Si-doped GaN therebetween. Also, a gate electrode G is formed on the surface of the undoped AlGaN layer at a location between the source and drain electrodes.
In the HEMT having such a structure, a piezoelectric field is produced due to a piezoelectric effect attributable to crystal strain at the boundary between the undoped GaN layer and the undoped AlGaN layer, the latter being constituted by compound crystal, with the result that a two-dimensional electron gas layer is formed directly under the heterojunction between the two layers.
While the source and drain electrodes S and D are in operation, the undoped AlGaN layer functions as an electron supply layer and supplies electrons to the undoped GaN layer. The electrons thus supplied travel at high speed to the drain electrode D by the action of the two-dimensional electron gas layer formed at the uppermost region of the undoped GaN layer. At this time, the gate electrode G is operated to form a depletion layer directly thereunder, thus enabling the device to accomplish various modulating operations.
In a preferred embodiment of the present invention, a GaN-based semiconductor device comprises: a base layer made of a GaN-based semiconductor material and having first and second surfaces; a bank made of a GaN-based first undoped semiconductor material and formed on the first surface of the base layer, the bank having a side wall surface and an upper surface; a thin layer made of a GaN-based second undoped semiconductor material having higher band-gap energy than the first undoped semiconductor material and formed on the side wall surface of the bank, the thin layer having a heterojunction with the first undoped semiconductor material; a first insulating layer formed on the first surface of the base layer; a gate electrode formed on the first insulating layer so as to be in contact with the thin layer; a second insulating layer formed on the gate electrode; a source electrode formed on the upper surface of the bank so as to extend beyond the heterojunction between the bank and the thin layer; and a drain electrode formed on the second surface of the base layer.
The bank formed on the first surface of the base layer may be one in number; alternatively, a plurality of banks may be formed. In order to permit heavy current to pass, however, multiple banks are preferably formed because, in this case, more two-dimensional electron gas layers can be formed.
According to another preferred embodiment of the present invention, a GaN-based semiconductor device comprises: a base layer made of a GaN-based semiconductor material and having first and second surfaces; a plurality of banks made of a GaN-based first undoped semiconductor material and formed on the first surface of the base layer, each of the banks having side wall surfaces on both sides thereof and an upper surface, those side wall surfaces of adjacent ones of the banks which face each other defining a U-shaped trench in cooperation with the first surface of the base layer; a thin layer made of a GaN-based second undoped semiconductor material having higher band-gap energy than the first undoped semiconductor material and formed on each of the facing side wall surfaces of the adjacent banks, the thin layer having a heterojunction with the first undoped semiconductor material; a first insulating layer formed on the first surface of the base layer in each of the U-shaped trenches; a gate electrode formed on the first insulating layer in each of the U-shaped trenches so as to be in contact with the thin layers facing each other; a second insulating layer formed on the gate electrode in each of the U-shaped trenches; a plurality of source electrodes each formed on the upper surface of a corresponding one of the banks so as to extend up to intermediate portions of the thin layers on both sides of the corresponding bank; and a drain electrode formed on the second surface of the base layer.
Each of the banks may extend in a straight line on the first surface of the base layer or may have a circular or rectangular or some other form as viewed from above the first surface. Also, the side wall surfaces of each bank may be either perpendicular to or inclined with respect to the first surface of the base layer.